> ==========
> bionet/mycology #4473, from permacltur at aol.com, 1850 chars, 17 May
2000 18:34:18 +0
> ----------
> Article: 7438 of bionet.mycology
> Xref: news.cix.co.uk bionet.mycology:7438
>Dan Hemenway said:
> Greetings:
> We have been having a discussion on the North American Fruit
Explorers list
> about mycorrhizal fungi. One contributor claims that the fungi
facilitated
> transfer of Rhizobia from her bean plants to nearby tomatoes where
they
> noduated the roots. She has had the tomatoes checked at a local
university
> which also said they were n-fixing nodules and was told, more or less:
> "Happens all the time."
>> Yet no one in the know I've been able to contact believes this is
possible.
> While I am n ot quite that absolute, I am skeptical. Does anyone on
this
> newsgroup have info that will dispell my skepticism?
>> Dan Hemenway
> Barking Frogs Permaculture Center, Sparr Florida USA
Hello Dan,
I have pondered this for a bit and think there a few things that can
(and maybe should) be said.I should say right off, I am very sceptical
too.
(1) Arbuscular mycorhhizal fungi (AMF) are pretty promiscuous so going
from bean to tomato would not be a problem.
(2) They are non-septate fungi so there are not quite as many barriers
to tranfer as would be true for a septate fungus.
(3) There are quite a few reports of bacteria within fungal hyphae (just
as there are of bacteria in apparantly healthy intact plant root cells.
So, potentially there is a mechanism there.
Further there are reports of Rhizobia infecting a non-legume
(Parasponia) and fixing nitrogen. No nodules are produced and the
bacterium does not change into the bacteroid form found in fixing
nodules. The Rhizobium in Parasponia remains within a network of
"infection threads" but (a big but) these "infection threads" could just
be fungal hyphae.
Problems seem to be that the Rhizobium Legume association is a pretty
sophisticated one, just getting a Rhizobium into the plant is very
unlikely to be enough. The University which said "it happens all the
time" is being extremely coy about its findings - they should be making
a fortune out of this discovery.
A few other thoughts. Producing bumps on roots is not quite enough.
Agrobacterium (a first cousin to Rhizobium) is renowned for producing
very nasty bumps.
How certain was "the University" about its recognition of Rhizobium ? A
colony on a plate is not much use.
How certain was "the University" about its measurements of Nitrogen
fixation ? What method did they use ? Acetylene-ethylene ? Isotopic
Nitrogen ? All of this takes some time and not a little effort.
On balance I remain unconvinced I'm afraid - but that is not the same as
uninterested.
Peter Harris
Reading,
UK.